Smoke generating compositions which are used by the military to produce red smoke typically comprise a formulation which includes sulfur as a fuel, potassium chlorate as an oxidizer, and red dye. Coolants such as magnesium carbonate and binder such as nitrocellulose or polyvinyl alcohol may also be included. Binders assist in maintaining the smoke generating formulation in a compact form so that it may be used in the production of, for example, smoke generating grenades.
The requirements for an effective colored-smoke composition include the following.
The composition must produce sufficient heat to vaporize the dye, as well as produce a sufficient volume of gas to disperse the dye into the surrounding environment.
The composition must ignite at a low temperature and continue to burn smoothly at a low temperature (well below 1000° C.). If the temperature is too high, the dye molecules will decompose, and the generated smoke's color quality and volume will deteriorate. The use of metal fuels should be avoided in colored smoke generating compositions because of the high reaction temperatures they may produce.
Although a low ignition temperature is required, the smoke-generating composition must be stable during manufacturing and storage over the expected range of ambient temperatures.
The molecules creating the colored smoke must be of low toxicity (including low carcinogenicity). Further, they must readily sublime without decomposition at the temperature of the pyrotechnic reaction to yield a dense smoke of good color quality.
In various contexts, it is desirable to have the capability to produce smoke suitable for a wide variety of applications. For example, the ability to produce smoke at a particular location may provide the basis for a remote signaling system. Such a system may have application in search and rescue operations and in military exercises. Smoke of a particular color and density may also be desirable for training purposes. For example, in order to train firefighters, it would be advantageous to simulate specific types of smoke produced by various fire conditions. For individuals working in a fire-prone environment, such as on an aircraft or ship, it would also be desirable to have the capability of simulating smoke produced by a fire in order to provide a realistic fire drill.
Smoke can be also used as a marker for various purposes. A smoke marker can be seen from substantial distances, both from the ground and from the air. Accordingly, a smoke marker would be useful in military operations, search and rescue, certain types of industrial projects, or in any other situation in which it is important to find and mark a particular location.
In a military context, the need for smoke-producing devices and compositions is well appreciated. Not only can smoke be used as a marker for search and rescue, but smoke may also be used to mark a particular target. It can also be used as a marker to determine the position of specific personnel and equipment.
Importantly, smoke can also be used to obscure vision. A smoke shield can be very helpful in conducting military operations in order to prevent adverse forces from obtaining a clear view of the operations. For example, it may be desirable to use a vision obscuring smoke in order to move troops and equipment under at least partial cover.
Various types of smoke-producing compositions and devices are presently known. It is known how to judiciously select the components of a smoke-producing composition that uses a sublimable organic coloring medium. By considering the kinetics of combustion and the desired yield of colored smoke, conventional smoke-producing compositions require a strongly exothermic composition but struggle to limit the degradation of the coloring medium by combustion. A weakly exothermic composition is unsuitable because it permits only a minimal percentage of coloring medium and furnishes only a very mediocre smoke yield that is hardly visible, especially when there is a cloudy or dark sky. A weakly exothermic composition is also difficult to ignite to initiate smoke emission.
It is also known that it is advantageous to adjust the combustion speed, because a rapid combustion provokes the destruction of the coloring medium or liberates the smoke too briefly or in a non-workable manner. In contrast, a slow combustion produces a smoke yield of minimal consequence and tends toward self-extinction of the combustion process. It is known to consider the effects of the reaction thermodynamics by making an appropriate choice of the components of the composition and by the conditions of compression of the composition.
However, most existing smoke-producing compositions have severe limitations. One of the limitations is that of toxicity. Many smoke-producing compositions incorporate materials that are severely toxic or become irritants when subjected to the heat necessary to produce smoke.
A variety of dyes have been used in colored smoke mixtures. Many of these dyes are presently under investigation for carcinogenicity and other potential health hazards.
The materials that work best in colored smokes have several properties in common:
Volatility: The dye must undergo a phase change to the gas state upon heating, without also undergoing substantial decomposition. Only low molecular weight dyes (less than 400 grams/mole) are usually used because volatility typically decreases as molecular weight increases. Salts do not work well: ionic species generally have low volatility because of their strong inter-ionic attractions within the crystalline lattice. Therefore, functional groups such as —COO— (carboxylate ion) and —NR3+(a substituted ammonium salt) should be avoided.
Chemical stability: oxygen-rich functional groups (—NO2; —SO3H) should be avoided. At the typical reaction temperatures of smoke compositions, these groups are likely to release their oxygen, leading to oxidative decomposition of the dye molecules. Groups such as —NH2 and —NHR (amines) are used, but potentially dangerous oxidative coupling reactions can occur in an oxygen rich environment.
Previous dyes for military markers and grenades are commonly polycyclic or aromatic amino, hydroxyl, azo and keto compounds, in which unsaturation conjugated with the aromatic structure. Such compounds are known to cause chromosome mutations, which may lead to cancer. Coloring agents for new munitions are chosen to minimize potential carcinogenicity, which increases their cost, but allows use and recycling of the dyes without high health risks.
The current M18 red smoke formulation uses potassium chlorate, sulfur and a red dye to generate a red smoke signal. While effective, the products generated are not environmentally friendly and toxicity testing on the red dye has been shown it to be more toxic compared to other dyes. Upon reaction the sulfur in the formulation creates toxic and environmentally harmful sulfur dioxide products which have known toxic characteristics. The sulfur also creates a safety issue during production, due to friction and impact sensitivity when intimately mixed with potassium chlorate.
Prior art colored smoke formulations have generally employed the use of mixtures containing a fuel, an oxidizer, and a dye. The principle behind the use of such formulations lies in the reaction between the fuel and oxidizer, and the accompanying release of a large amount of energy during the reaction. The exothermic reaction releases the energy contained in the bonds of the highly structured fuel molecule as heat. This causes the dye component of the formulation to undergo a series of phase transitions from a solid to a liquid and ultimately to a gas. However, if the temperature of the fuel-oxidizer reaction is too high, the dye will degrade, and the quantity and color quality of smoke generated will be unsatisfactory.
Conventionally, the dye exists as a solid crystal at standard temperature and pressure. When heat generated by a fuel-oxidizer reaction is applied to the solid crystal, dislocations of the molecules occur within the crystalline lattice. As molecules of the dye become detached from the central lattice, a liquid is formed. As more heat energy is applied, the individual molecules of the dye begin to move more rapidly, and this molecular activity is responsible for the transition of the dye molecules from the liquid phase to the gas phase.
Although heat is required for the dye to undergo the essential phase changes to produce colored smoke, individual molecules of the dye are also subject to degradation at elevated temperatures. If the molecular structure of the dye is subject to forces and energies that are great enough to cleave the molecule's bonds, changes in smoke color or loss of color properties are likely to occur.
Other known compositions have the drawback of burning at temperatures that are too high (600 to 800° C.) or leave too many carbonaceous residues, which are impermeable to the dye molecules in the gas phase. These conditions cause the destruction of the smoke-producing components and therefore demonstrate poor effectiveness in producing colored smokes. Another drawback may be the rapid ascent of the smoke because of the smoke's high temperature, which causes the smoke to dissipate too quickly for the desired effect to be achieved.
Various smoke generating compositions have been made in the past. US 2015/0329437 to Hultman discloses a pyrotechnic composition including a fuel, an oxidizer, flow rate control agents and oleoresin capsicum as an irritant. The composition is useful in crowd control products. The composition comprising approximately 13% to 24% potassium chlorate, 3% to 22% baker's sugar, 6% to 22% magnesium carbonate, 20% to 40% terephthalic acid, 2% to 28% dye, 1% to 25% magnesium stearate as the rate controlling agent, and 20% to 35% nitrocellulose as the binding agent. However, the dye composition is not addressed, and it is uncertain that such composition would meet the M18 smoke grenade standards, be less toxic, and having a controlled burn rate.
GB 1139761 to Cross discloses a riot-controlled composition comprising 40% of o-chloro benzal malonitrile, 27% potassium chloride, 18% of sugar, 12% of magnesium carbonate, and 3% of cellulose nitrate. Similar to Hultman, the dye ingredient is not addressed, and it is uncertain that such composition would have controlled burn or meet the M18 grenade standard.
RU2369590 to Sergiveech, et al. discloses pyrotechnic composition of color smoke containing potassium chlorate as an oxidizer, thiourea as fuel—gasifier, dicyandiamide, fluorine rubber SKF-32, lamellar graphite of Piv grade as organic dye-monomethyl aminoanthraquinone, red C fat-soluble dye, fat-soluble red anthraquinone or hexachlorobenzene dye.
U.S. Pat. No. 6,558,487 to Tadros, et al. discloses a smoke generating composition including at least one smoke generating material and an effective amount of a polymerized monosaccharide or disaccharide both as a binder and a fuel. In the preparation of colored smoke generating compositions, potassium chlorate, magnesium carbonate and sucrose are typically combined with a dye such as solvent yellow 33 (for the production of yellow smoke) and solvent yellow 33 combined with solvent green 3 (for the production of green smoke). Further, from about 12 to 100% of the sucrose is polymerized, formed into a solution and then sprayed over the remaining components to bind the components together into a smoke generating composition. U.S. Pat. No. 6,558,487 does not address the issue of controlled burn, or the toxicity associated with the dye.
US 2014/0238258 to Stoenescu discloses a colored pyrotechnic smoke-producing composition that has an oxidizer, a sugar fuel, a flame retardant, a dye, a coolant, and a binder. The flame retardant is one or more nitrogen-rich compounds. The composition may be in pelletized form, or in the US form of a solid charge. Similar to other prior art, US 2014/0238258 also does not address the issue of controlled burn, or the toxicity associated with the dye.
However, many such compositions are also corrosive and damaging to both electronic and mechanical equipment. Finally, some compositions produce an excess of heat and flame, again limiting their usefulness and requiring that additional safety measures be taken. For these reasons, conventional smoke-producing compositions are found to be inadequate.
Therefore, there is a need to create comparable signaling color and meet the burn time requirements from the M18 grenade specification. Specifically, there is a need to replace existing red dye with at least one red dye that has been shown to be less toxic. There is also a need to replace the sulfur fuel with an environmentally friendly and less toxic alternative.